1. Field of the invention
This invention relates to a method for improving the fuel economy of an internal combustion engine, and to a synergistic blend of friction modifiers which may be added to the crankcase lubricating oil of an internal combustion engine for accomplishing such result. The synergistic blend of friction modifiers comprises the combination of (a) an amine/amide friction modifier formed, for example, by reacting a carboxylic acid, such as oleic acid and/or isostearic acid with an amine, such as diethylene triamine or tetraethylene pentamine, and (b) an ester./alcohol friction modifier such as glycerine monooleate and/or glycerine monoricinoleate. The enhanced friction reduction achieved by the use of the synergistic blend of friction modifiers allows the formulation of engine lubricants which meet Tier II fuel economy.
2. Discussion on the Prior Art
The constant threat of diminishing sources of fossil fuels and the resulting increases in prices for such fuels, coupled with the federally mandated requirements for reducing the amount of toxic emissions spewed into the atmosphere, has resulted in a great deal of interest in improving fuel economy, particularly the fuel economy of automobile combustion engines.
Such interest has lead to the discovery of cleaner burning compositions, as well as to the discovery of a variety of fuel and/or engine lubricating oil compositions which result in improved fuel economy, that is a higher number of miles obtained in a given vehicle per gallon of fuel.
One such discovery, which is described in U.S. Pat. No. 4,584,112, involves lubricating the crankcase of an internal combustion engine with a lubricating oil composition consisting essentially of a hydrocarbon oil of lubricating viscosity, from 15 to 25 millimoles per kilogram of zinc 0,0-di(2-ethylhexyl) phosphorodithoate, and from 0.25 to 2 wt. % of pentaerythritol monooleate.
U.S. Pat. Nos. 4,492,640 and 4,492,642 also describe methods for reducing the fuel consumption in internal combustion systems. Both of these patents described the addition to lubricating and/or fuel compositions used in an internal combustion engine of a friction reducing compound. The friction reducing compound disclosed in U.S. Pat. No. 4,492,640 comprises a boron derivative of a mixture of alkoxylated alcohols and hydroxy sulfides, whereas the friction reducing compound disclosed in U.S. Pat. No. 4,492,642 comprises the product formed by reacting a borating agent with an ammoniated hydrocarbyl epoxide.
U.S. Pat. No. 4,512,903 discloses lubricating compositions which contain still other friction reducing compounds, namely, amides prepared from mono-or polyhydroxy-substituted aliphatic monocarboxylic acids and primary or secondary amines.
The use of a lubricating oil composition comprising a base oil and a friction modifier in an automatic transmission or a continuously variable transmission, or as a lubricating oil for use in parts including wet clutches or a wet brake of an agricultural tractor is described in European Application 286,996. The friction modifying compounds used in the lubricating oil compositions of that European application are said to include such friction modifiers as phosphoric acid esters, phosphorus acid esters, amine salts of phosphoric acid esters, amine salts of phosphorus acid esters, sorbitan fatty acid esters, pentaerythritol fatty acid esters, glycerine fatty acid esters, trimethylolpropane fatty acid esters, glycol fatty acid esters, carboxylic acids, carboxylic acid amides, carboxylic acid esters, metal salts of carboxylic acids, fats and oils, higher alcohols and sulfur-containing compounds. The numerous disclosed friction modifiers can be used alone or in combination with one another. This European application states that the lubricating oil compositions exhibit excellent frictional characteristics, which change little with time and which are characterized by excellent stability against oxidation. However, there is no suggestion of using the disclosed lubricating oil compositions as a crankcase lubricant, nor is there any discussion of any possible effect on fuel economy.
A similar disclosure is contained in Japanese Kokai 60-173097. This Japanese publication discloses lubricating oil compositions which are useful in automatic transmissions and which comprise a base oil and a friction modifier. The lubricating oil compositions exhibit high stability against oxidation and change of properties with time, exhibit low shift shock and high transmission torque, and anticorrosive qualities. The friction modifiers which are disclosed in the Japanese publication this document include phosphoric acid esters, phosphorus acid esters or an amine salt thereof; a fatty acid ester of sorbitan, pentaerythritol, glycerine, trimethylolpropane or a glycol; a carboxylic acid or an amide, ester or metal salt thereof; and fats or oils, higher alcohols or sulfur-containing compounds. There is no discussion in the Japanese publication of the possibility of adding the lubricating oil compositions to a crankcase of an internal combustion engine, nor is there any discussion of fuel economy.